Two way RF transceivers and one way RF receivers are well known in the art. Through use of such devices, a user can receive messages from one or more distant locations. To facilitate such communications, such radios are often equipped to selectively receive more than one channel. So configured, a user can choose to listen and communicate on a variety of channels depending upon his needs at a particular moment in time.
In general, however, at least one of these channels will typically comprise a priority communications channel. High priority messages intended to be heard by the user will ordinarily be transmitted on that priority channel. Since such messages are, by definition, of a priority nature, a need exists for the user to hear such messages when they occur. An obvious problem arises in accomplishing this end result in a multiple channel radio where the user may be communicating on a non-priority channel coincident with transmission of the priority message.
One very simple prior art solution has been to continuously repeat the priority message until an acknowledgment has been received from all pertinent parties. This approach is very wasteful of time and communications spectrum.
Another proposed solution requires multiple receiver radios (as versus multiple channel radios) such that the priority channel can constantly be monitored by one receiver while the user listens to another channel on another receiver. If a priority message occurs, the user will then be assured of receiving it. This solution, of course, requires redundant receiver structure and consequently constitutes a relatively high cost solution.
Another solution has been to periodically monitor the priority channel while listening to the non-priority channel to ascertain whether a carrier is present. Such monitoring can be accomplished in a relatively short time window, which time window can be made short enough (such as, for example, 30 msec or less) to be virtually unnoticeable by the user. Then, if the receiver detects a carrier on the priority channel, the receiver can switch from the non-priority channel to the priority channel to allow reception of the priority message.
In a system dedicated to the use of only one particular group, such a solution constitutes an adequate remedy. Unfortunately, given the paucity of spectral resources as compared to the demand for radio communications, such single group systems are not the rule. Instead, such channels typically host a number of groups. In order to allow such groups to remain in contact only with one another and avoid unnecessary communications to users not in the group in question, various squelch control mechanisms are used. One such mechanism, offered by Motorola, Inc. under the trademarks Digital Private Line (DPL), a subaudible digital word comprised of 23 bits (12 of which are actual information bits and the remainder of which are provided for parity, error correction and the like) are transmitted on the priority channel. The code word so transmitted can be received, decoded, and compared in a receiver against a stored code word or words, such that the receiver can ascertain whether the message associated with that transmitted code word is intended for its reception. If reception is appropriate, the radio will unsquelch and the message will be rendered audible. If the code word does not match one of the stored code words, then the radio will not unsquelch and the user will not be bothered by the message.
Unfortunately, such code words typically take 171 milleseconds or more to transmit in full. Therefore, a full code word cannot be received within the narrow time window constraints of the prior art approach noted above. This being the case, a need has arisen for priority channel scanning for DPL and similar encoded systems, which scanning does not unduly interfere with communications concurrently taking place on a non-priority channel.